This invention relates to indexable cutting inserts for face milling and more particularly to inserts adapted to face milling applications, such as the milling of cast iron, which are tolerant of negative rake-face geometry.
In such applications, it is possible to employ inserts of the simplest form, viz., a rectangular parallelepiped having two opposed major faces which are square and which may be indexed so as to selectively present each of four narrower surrounding minor faces as rake faces, and to present each of the eight long edges of the two major faces as the major cutting edges of the insert.
An insert of this type is shown by Lacey U.S. Pat. No. 3,818,562 in which the juncture of the two active cutting edges of each rake face is strengthened by a chamfer created by grinding a triangular facet which extends rearwardly from the rake face along a minor portion of that edge of the exposed major face of the insert which trails from the rake face and provides clearance behind the chamfered cutting edge.
It has been discovered, however, that the provision of corner relief by the geometry proposed by Lacey requires very negative radial rake in order to provide the minimum clearances behind the cutting edges required by good practice, particularly in applications for which relatively low lead angles of the face mill are desired. It is well understood in the art that the more negatively radial the rake face must be disposed to obtain the necessary clearance angles for reasonable tool life, the greater is the force required for the milling operation and thus the greater the heat produced in the cutting insert, with consequent shortening of tool life for that reason. Equally important, however, to many cutting applications is that the more negatively radial the rake face is disposed, the greater is the distortion of the workpiece from the cutting forces, and the greater the incidence of "break out", i.e., chipping, at the exiting edge of the resulting surface of the workpiece being milled.
The present invention is based upon the discovery that by a relatively minor change in the method of producing the strengthening chamfer at the corner of the two cutting edges of the rake face, a very substantial increase in tool life can be achieved as a result of the cooperation of two factors made possible by the invention, namely, the achievement of adequate clearances at low lead angles with much lower angles of negative radial rake, thus reducing the cutting forces required, and the increase of the clearance angle behind the chamfered portion at the corner of the two cutting edges, thereby increasing the life of that vulnerable portion of the cutting edge.
The result has been to substantially increase the life of the insert in each of its eight positions, and to significantly reduce the cost of tooling for face milling operations of the kind described, particularly for heavy rough-cutting operations, with resulting reduction of downtime for tool replacement and in the overall operating costs attendant thereto.